Years ago I was in one based on a cruise liner somewhere down near Pax Rulin on Imperial border. My gaming group also ran a freeform (the Oz equivalent of a LARP) based in Hitler's bunker in the last three hours of WW2.

My advice is
1. Concentrate on creating good, playable, interesting characters.
2. Give them plenty of "hooks" (i.e. in-game goals) that force them to interact with all the other characters.
3. Create an overall "story arc" and a method of pushing it forward; for example, use multiple referees as NPCs who can drive the plot forward through some pre-planned, set-piece actions.
4. Read Tavonni Repair Bays --> Other Assorted Notes --> Designing Convention Modules on my website. These are a set of dot-points you can follow while prepping the scenario.